A large quantity of titanium dioxide of both the anatase and rutile types has long been manufactured and consumed as a white pigment. The pigment-grade titanium dioxide has an average particle size from about 0.1 μm to about 0.3 μm and exhibits a high hiding power compared to other white pigments. Titanium dioxide particles having a diameter less than 0.1 μm exhibit a unique property of selectively blocking the passage of ultraviolet rays while transmitting visible light. By virtue of this property, they are formulated in cosmetic preparations for preventing suntan. Much attention has been drawn in recent years to a high photocatalytic activity of titanium dioxide. Anatase-type titanium dioxide is mainly used for this purpose owing to its higher photoactivity than other types. Sols of anatase-type titanium dioxide have been used for forming a coating film having photocatalytic function on a substrate. Various production methods and usage of the photocatalyst film are described in many patent documents.
The titanium dioxide sols also find use as a hard coat or an anti-reflection film on optical elements requiring high transparency, high adhesion strength with substrates, high refractive index and high scratch resistance. Sols of anatase-type titanium dioxide have been mainly used for this purpose owing to their higher transparency than rutile-type titanium dioxide sols. However, the anatase-type titanium dioxide sols are disadvantageous in that they act on an organic material such as plastics to decompose and color due to high photocatalytic activity. While the photocatalytic activity may be reduced by coating the sol particles with a hydrated metal oxide, this coating compromises the transparency of the sol. It is desirable to provide a surface-treated titanium sol having decreased photocatalytic activity while retaining the transparency of the sol before surface treating in order to expand the application field of the titanium dioxide sols.
Methods for producing the titanium dioxide sol are known in the art. The method generally comprises the steps of hydrolyzing a water-soluble titanium salt, neutralizing and washing the hydrolyzate to produce hydrated titanium dioxide. The sol is prepared from hydrated titanium dioxide by (1) peptizing with hydrochloric or other strong acids, or (2) dissolving as complex ions by adding complexing agents such as hydrogen peroxide and then heating the solution. The titanium dioxide sols prepared by the above method are either anatase type or amorphous type. Rutile-type titanium dioxide sols are generally produced by treating the hydrated titanium oxide prepared by the neutralization of a water-soluble titanium salt with an alkali, and aging with an acid. The resulting rutile-type titanium dioxide sol is not comparable to the anatase-type titanium dioxide sol in terms of transparency because removal of salt contaminants produced in the production process is hardly possible.
Titanium dioxide particles, irrespective of anatase or rutile type, contacting with organic materials such as plastics require to be coated with a photocatalytic activity-free hydrated metal oxide to retard or decrease their photocatalytic activity. Example of metal oxides that have been proposed for this purpose include SiO2, ZrO2, SnO2, WO3, Al2O3, Sb2O5 and their double oxides. See, for example, JP 63/185820A, JP 04/214028A, JP 10/245225A, JP 10/310429A, JP 10/158015A, JP 2000/53421A, JP 2000/62119A, JP 2001/122621A, JP 2002/363442A and JP 2006/306980A. However, coating the titanium dioxide sol with these metal oxides compromises high transparency and high refractive index inherently possessed by the titanium dioxide sol. Furthermore, the resulting products are not satisfactory in light resistance or stability. A need exists for provision of surface-coated titanium dioxide sols free from the above disadvantages.